Peripheral blood lymphocytes grown in the presence of high doses of interleukin-2 (IL-2) acquire the ability to lyse tumor cells. This cytolytic activity is not restricted by antigens encoded in the major histocompatibility complex (MHC) and is observed with a variety of both autologous and allogeneic tumors. This activity has been called lymphokine-activated killing (LAK) and has been used in the clinical treatment of tumors. The origin of cells with LAK activity and the mechanism by which they recognize and kill their targets are unknown. Most of the cells with LAK activity display the CD3-CD4-CD8-CD16+ phenotype. Populations of cells with this phenotype were purified by a combination of antibody-mediated complement killing and cell sorting. Clones were also obtained by limiting dilution. RNA hybridizations demonstrated that these cells express only the CD3-epsilon gene of the CD3 complex and the delta gene of the TCR complex. The TCR delta transcripts were derived from unrearranged genes. These observations are consistent with the notion that these MHC-unrestricted cytotoxic cells are derived from an early stage in the T cell lineage. An antibody has been isolated that binds to a cell surface molecule on these cells and that can modulate their cytotoxic activity. A cloning strategy based on cDNA expression is in use to isolate the gene encoding this cell surface molecule.